To create an image of a specimen by means of a scanning electron microscopy, a sample is scanned by an electron beam. Secondary electrons that are emitted from the sample by the striking electron beam are detected and by this signal a synchronously scanned electron beam in a monitor is modulated. The scanning usually takes place within a vacuum, especially to enable the action of both the electron gun and the electron detector.
The handling of a specimen within a vacuum presents a lot of problems. The biological specimens not survive within a vacuum, wet specimens evaporate before an exact image can be produced. An observation of specimens from which different gaseous substances may leak within a vacuum of the microscope should be carefully considered in advance. Nonconductive samples within a vacuum accumulate static charges on their surface, making thus precision microscopy practically impossible. This problem would be solved by coating such specimens, which, in some specimens such as semiconductors, may entail their destruction and thus it is practically impossible to carry out their nondestructive analysis.
A series of experiments has been carried out to observe specimens in which the vacuum of the microscope specimen chamber was separated from the source of the electron beam to enable the observation of the specimen while maintaining sufficient vacuum in the region of the electron gun. U.S. Pat. Nos. 4,785,182 and 4,880,976 by James F. Mancuso et al. describe a secondary electron detector to be used in gaseous atmosphere. In this case the vacuum of the electron gun and electron microscope column is established to the value common in the electron microscopy. This part of the electron microscope is then separated from the specimen chamber by a pressure reducing diaphragm, which is substantially a lid enclosing this part of the electron microscope. A small orifice is made in the middle of the lid enabling the passage of the electron beam, nevertheless, it establishes considerable resistance to the flow of gas from the specimen chamber to the electron gun. When high-efficiency vacuum pumps are used, the vacuum can be maintained at a value acceptable for the operation of the electron gun and in turn the gas pressure in the specimen chamber can be maintained at the level required by the specimen or sample. Between the sample and the pressure-limiting diaphragm a plate electrode with an orifice in the middle is located to enable the passage of the electron beam, to which electric potential is applied. The secondary electrons emitted from the sample are attracted by the electric field of the electrode and conveyed to the detector.
The evident disadvantage this arrangement suffers is the impossibility of simultaneously optimizing the pressure within the specimen chamber and the pressure in the detector chamber. If, in consideration of the biological samples and their survival within the vacuum, the pressure is maintained at a higher level, the conditions necessary for the operation of the detector are generally insufficient and, vice versa, if the vacuum in the specimen chamber is established to an optimum value for the detector operation, the biological samples do not survive and no non-destructive observation can be performed.
WO 98/22971, applied for by Leo Electron Microscopy Limited, describes another system, in which negative voltage is applied to a sample holder and this voltage repels secondary electrons emitted from the sample by the primary electron beam into a collision zone in a specimen chamber, in which the collisions of the accelerated secondary electrons and gas molecules in the gaseous medium triggers a cascade of collisions and thus generate an amplified signal of secondary electrons, which is detected in the microscope. This signal comprises photons generated by the collisions of the accelerated secondary electrons and the gas molecules in the gaseous medium and it is detected by a photo-multiplier, to which the photons are conveyed via a light guide.
Nevertheless, this system has not solved thoroughly the problem of choking the secondary electrons signal by the back-scattered electrons signal. In addition to it, the photon signal is usually very weak and the quality of the produced image is low.
UK Patent No. 2367686 by Leo Electron Microscopy Limited, describes another system that uses a detector chamber having a different vacuum value than the specimen chamber, both chambers being separated by a thin Al foil that should prevent the gas from the specimen chamber to penetrate to the detector chamber while at the same time enable the electrons to pass through the foil from the specimen chamber to the detector chamber.
The disadvantage of this system is that it does not sufficiently differentiate between secondary electron signal that gives a topographical information and a backscattered electrons signal that gives an information about the material of the specimen. Namely, a backscattered electron easily penetrates the Al foil and causes the emission of the secondary electrons from the surface of the foil that faces the scintilator. These electrons are then attracted by the scintilator and the signal with a topographical information is thus mixed with a signal that bears an information about the material of the specimen. Further mixture of both types of information occur if the backscattered electron impinges, after passing through the Al foil, directly the scintilator. Another disadvantage is that the Al foil is too thin, about 7,5 nm, and is thus vulnerable to a damage in use. Further disadvantage is that the foil is in the specimen chamber environment easily contaminated and considering how thin the foil is, the contamination layer acquires soon a thickness comparable with the thickness of the foil, what results in worse penetrability of low energy secondary electrons without influencing high energy backscattered electrons and thus in a worse efficiency of signal collection.